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Date:   Thu, 9 May 2019 07:59:15 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     "Reshetova, Elena" <elena.reshetova@intel.com>
Cc:     David Laight <David.Laight@ACULAB.COM>,
        Andy Lutomirski <luto@kernel.org>,
        Theodore Ts'o <tytso@mit.edu>,
        Eric Biggers <ebiggers3@gmail.com>,
        "ebiggers@google.com" <ebiggers@google.com>,
        "herbert@gondor.apana.org.au" <herbert@gondor.apana.org.au>,
        Peter Zijlstra <peterz@infradead.org>,
        "keescook@chromium.org" <keescook@chromium.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "jpoimboe@redhat.com" <jpoimboe@redhat.com>,
        "jannh@google.com" <jannh@google.com>,
        "Perla, Enrico" <enrico.perla@intel.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall
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* Reshetova, Elena <elena.reshetova@intel.com> wrote:

> > * Reshetova, Elena <elena.reshetova@intel.com> wrote:
> > 
> > > CONFIG_PAGE_TABLE_ISOLATION=n:
> > >
> > > base:                                  Simple syscall: 0.0510 microseconds
> > > get_random_bytes(4096 bytes buffer):   Simple syscall: 0.0597 microseconds
> > >
> > > So, pure speed wise get_random_bytes() with 1 page per-cpu buffer wins.
> > 
> > It still adds +17% overhead to the system call path, which is sad.
> > Why is it so expensive?
> 
> I guess I can experiment further with buffer size increase and/or 
> using HW acceleration (I mostly played around different rdrand paths now). 
> 
> What would be acceptable overheard approximately (so that I know how
> much I need to squeeze this thing)? 

As much as possible? No idea, I'm sad about anything that is more than 
0%, and I'd be *really* sad about anything more than say 1-2%.

I find it ridiculous that even with 4K blocked get_random_bytes(), which 
gives us 32k bits, which with 5 bits should amortize the RNG call to 
something like "once per 6553 calls", we still see 17% overhead? It's 
either a measurement artifact, or something doesn't compute.

Thanks,

	Ingo